Bias Temperature Instability (BTI) refers to the phenomenon that occurs with respect to certain semiconductor devices and is considered one of the most critical elements for reliability. It is particularly noticeable under conditions of a negative bias, elevated temperatures, and long term operation. In the silicon (Si) semiconductor field, this long-standing BTI problem has manifested itself for many years and there has been considerable research and multiple designs to mitigate the problem in Si devices. In the fast-growing silicon carbide (SiC) field, the bias temperature instability is generating major concerns for reliability, performance limitations, and product qualification. For example, a strong Negative Bias Temperature Instability (NBTI) has been observed in SiC devices, resulting in a significant decrease in the absolute threshold voltage, to the point that a normally-off device becomes normally-on (conductive with gate-source voltage at zero volts). The NBTI problem has been documented, however there has yet to be an industry solution.
While the BTI issue has been alleviated in many instances in the Si device marketplace or is less of an impact for Si, there are significant behavioral differences between Si and SiC devices such that the mechanisms used to alleviate the problem in Si do not readily translate to SiC.
While the SiC community is yet to reach a consensus about the root cause of the NBTI problem, it is generally attributed to the existence of interface traps and oxide charges, and can be induced by operating devices at high temperature, and under a bias condition for extended periods. Regardless of the cause of BTI, the effect is clearly demonstrable. As an example, for metal-oxide semiconductor (MOS) devices that operate with a negative bias applied on the gate-to-source, the effects of NBTI are evident by a decrease in the threshold voltage. The threshold voltage instability is more noticeable when a device is under a negative bias and subject to elevated temperatures. As a further example, metal-oxide-semiconductor field effect transistors (MOSFETs) including silicon carbide MOSFETs, experience a shift in the threshold voltage when subjected to combined voltage and temperature stressing. Thus, threshold voltage shift and NBTI have raised reliability concerns, hampering the product adoption, especially the introduction and exploitation of SiC devices into new market applications where SiC devices have unique operating characteristics, can operate at higher temperatures, and enable novel applications.